Enceladus Plume Hydrogen (2017): Signs of Hydrothermal Activity

Hunter Waite and the Cassini team found molecular hydrogen in the plume of Saturn’s moon Enceladus in 2017. It is best explained by warm water reacting with rock beneath the moon’s hidden ocean, hydrothermal activity like Earth’s deep-sea vents. The ocean holds chemical energy microbes could use.

The question

Enceladus is a small, icy moon of Saturn. NASA’s Cassini spacecraft, in orbit around Saturn from 2004, discovered jets of water vapor and ice grains spraying from cracks near its south pole, and earlier flybys pointed to a global ocean under the ice, resting on a rocky core. Water and organic molecules were known. The missing piece for habitability was energy: could anything in that dark ocean power life?

What the paper found

• Hydrogen in the plume. Molecular hydrogen made up 0.4 to 1.4% of the plume gas.
• The rest of the recipe. The plume was mostly water vapor (96 to 99%), with carbon dioxide (0.3 to 0.8%), methane (0.1 to 0.3%) and ammonia (0.4 to 1.3%).
• A rocky source. The hydrogen is best explained by ongoing reactions between warm water and the moon’s rocky core, in other words hydrothermal activity. The team judged it unlikely to be left over from the moon’s formation or made in the ice shell.
• Food for microbes. On Earth, microbes called methanogens combine hydrogen with carbon dioxide to make methane and gain energy. Enceladus’s ocean has both ingredients.

How they did it

The measurement came from Cassini’s Ion and Neutral Mass Spectrometer (INMS), an instrument that sorts gas molecules by mass to identify them. On 28 October 2015, during its final pass through the plume, Cassini flew just 49 kilometers above the south polar surface, a little farther than the length of a marathon.

The hard part was proving the hydrogen came from Enceladus and not from the instrument itself. The team developed a special operating mode to minimize and measure spurious hydrogen, backed by simulations and laboratory tests, before concluding how much was truly native to the moon.

Why it mattered

The result showed that Enceladus’s ocean has not only water and organics but also available chemical energy, a key requirement for life as we know it. The team was careful: they found no evidence of life itself, only conditions that could support it.

The approach echoes a classic of astrobiology. The Miller-Urey experiment of 1953 asked whether simple chemistry plus energy could make life’s ingredients; Cassini found a real ocean where that chemistry may be running today.

What we know now

Cassini ended its mission on 15 September 2017. Its archived data kept producing results: in 2023 a team led by Frank Postberg found phosphates in ice grains from the same ocean, completing the set of six key elements for life inferred in that ocean. Enceladus is now a leading candidate for a future life-detection mission.

Who wrote it

J. Hunter Waite of the Southwest Research Institute in San Antonio, Texas, led the study as principal investigator of Cassini’s INMS. Co-authors included Christopher Glein, a specialist in the chemistry of alien oceans, and Rebecca Perryman, who led INMS operations. The paper appeared in Science on 14 April 2017.

Read the original

Waite, J. H., Glein, C. R., Perryman, R. S., et al. (2017). “Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes.” Science 356, 155-159. doi:10.1126/science.aai8703

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